4. OPTICAL COUNTERPARTS OF RADIO SOURCES IN THE HDF

Figure 3 shows the distribution of
IAB magnitudes of the 13 radio sources in the HDF.
Of the seven sources found in the complete sample, all have optical
counterparts brighter
than IAB = 21.2. At least half of the optical identifications
are with spiral or irregular
galaxies, many of which appear to be merging or are in small groups. The
remaining
optical counterparts are composed of nearby field spirals, red
ellipticals, and possibly a
few late type AGN. There are no identifications with quasars or with
galactic stars.

Figure 3. Magnitude distribution for radio
sources in the HDF. The black areas refer to the
complete sample of seven sources and the lightly shaded areas the six
additional radio sources
not found in the complete sample. IAB magnitudes are taken
from Williams et
al. 1996.

Spectroscopic or photometric redshifts are available for all of the
galaxies in the HDF
with detected radio emission. All have redshifts less than 2.01 and the
seven identified
sources in the HDF complete sample have redshifts less than 1.01
(Figure 4). Although
these microjansky radio sources are a million times weaker than Jansky
level sources, such as found in the all sky 3CR or Parkes surveys, it is
important to note that the
redshift distribution of the microjansky sources does not differ
appreciably from that of the strong radio source population.

Figure 4. The redshift distribution for
radio sources in the HDF. The black areas refer to the
complete sample of seven sources and the open areas the six additional
radio sources not found
in the complete sample which are identified with galaxies having
measured redshifts. Redshifts
are taken from the Keck/HDF Consortium
(Moustakas et al. 1997).

This is because the radio luminosity function is relatively
steep. Unlike the optical
galaxy counts which reach to successively more distant galaxies at faint
magnitudes, the
radio counts, at microjansky levels, sample roughly the same part of
redshift space as
the strong source samples, but reflect the distribution of a much lower
luminosity group of radio sources.

None of the galaxies in the HDF are strong FRII radio galaxies. As shown
in Figure 5,
the radio luminosity of all of the galaxies in the HDF is less than 6 x
1024 W/Hz at
8.5 GHz, typical of FRI radio galaxies, weak AGN found in some
elliptical and Seyfert
galaxies, and other galaxies with active star formation. The weakest
source in the field,
HDF 3648+1427
(we denote radio sources in the HDF and HFFs by their coordinates
truncating the 12 hours and +62 degrees), which is identified with a
bright (IAB = 18.4)
relatively nearby (z ~ 0.02) galaxy which has a luminosity of only 2 x
1019 W/Hz,
comparable with that of normal spiral galaxies such as M31.

Figure 5. The luminosity distribution of
radio sources in the HDF calculated assuming
H0 = 50 km/sec/Mpc and q0 = 1/2. The dark areas
refer to the complete sample of seven
sources and the open areas to the six additional radio sources not in
the complete sample which have measured redshifts.

Only one source in the Hubble Deep Field (HDF 3646+1404) shows clear
evidence of
variability over the approximately 18 month period covered by our
observations. It is a
relatively powerful radio source with a luminosity at 8.5 GHz of 3 x
1024 W/Hz and has
an unresolved radio core less than 0.1 arc seconds in diameter and a
flat radio spectrum.
It is identified with a face on Sb galaxy at a redshift of 0.96 which is
very red in color and contains a compact optical nucleus with broad
emission lines
(Phillips et al. 1997).
It is bright in the infrared H and K bands
(Cowie et al. 1997).
Rowan-Robinson (1997)
interpret the ISO 6.7 micron flux density of 52 µJy as evidence for
a massive starburst
with a SFR of about 200 M0/yr. However, the observed radio
variability combined with
its flat radio spectrum and small size suggests that the radio emission
in HDF 3644+1404
originates in an AGN, and is not primarily due to star formation.

The strongest radio source (S = 600 µJy) in the Hubble Deep Field,
HDF 3644+1133, is
identified with a bright (MB = -23) red elliptical galaxy at
a redshift of 1.01. The radio
luminosity is 6 x 1024 W/Hz, typical of the stronger FRI
radio galaxies. It has a strong
flat spectrum radio core which is unresolved at our highest angular
resolution and is less
than 0.1 arc seconds in diameter. A double lobe structure reaches some
15 arc seconds
to the north and south of the unresolved core. HDF 3644+1133 has also
been observed
by ISO and has a 6.7 micron flux density of 50 µJy (Goldschmidt et
al. 1997). Curiously,
there is a very blue apparently elongated chain of galaxies, or possibly
merging system,
which lies within the northern radio lobe of HDF 3644+1133 and is at
essentially the
same redshift. This feature is similar in appearance to the "chain"
galaxies described by
Cowie et al. (1995),
although it appears much larger in extent than the
galaxies discussed
by Cowie et al. It is not clear what relation exists, if any, between
HDF 3644+1133 and
the "chain" galaxy which has multiple apparently unresolved "hot spots."
Possibly, the
"chain" galaxy is undergoing active star formation induced by the jet
emerging from
the red elliptical. We have detected radio emission from another "chain"
galaxy, HDF 3652+1354, but which is not in our complete
radio sample.

Interestingly, there is one source which appears just below our
completeness level with
a snr of 4.7 which is also seen in our 20 cm data with a snr of 3.5, but
which has no
optical counterpart down to the limiting magnitude of IAB =
27.6 of the HDF. If real,
it is unlikely that this unidentified source is an intrinsically faint
galaxy at moderate
redshift as no other radio sources are known to be identified with such
faint galaxies.
The observed radio emission could be the displaced radio lobe of an
extended source with
asymmetric structure and no detectable radio emission from the parent
galaxy, but we
consider this also unlikely as other microjansky sources all appear
coincident with their
optical counterpart. More likely, it may be a very high redshift (z > 6)
I dropout galaxy,
in which case H and J band observations with NICMOS may be able to
detect the parent galaxy.